1. Field of the Invention
The present invention relates to an image forming apparatus which forms a color image or a monochromatic image on a recording sheet, and a network system provided with the image forming apparatus. Particularly, it relates to a technology to reduce a delay in sheet feeding.
2. Description of the Related Art
In color image forming apparatuses such as a printer apparatus, a copying machine, and a facsimile apparatus, there is known a color tandem type where a plurality of developing devices contain different color toner particles, e.g., cyan, magenta, yellow, black, respectively, and individually develop latent images formed on a photoconductive drum, toner images of respective colors are transferred successively to a transferring belt, and the toner images are transferred from the transferring belt to a recording sheet, and fixed by a fixing device.
In the case of requiring high speed image forming, particularly, an image forming apparatus of the color tandem type having a transferring belt is employed much more. In a color tandem type image forming apparatus, a latent image forming process is started prior to a sheet feeding operation, and the sheet feeding operation is started during the time when the transferring belt receives toner images. To regulate the timing of conveying a recording sheet to an image transfer position where a toner image is transferred from the transferring belt to a recording sheet, a pair of registration rollers temporarily holds the recording sheet for a predetermined time (waiting time), and then conveys the recording sheet. The waiting time is set at a time not to cause jamming of a recording sheet at the registration roller.
Further, the timing of starting the sheet feeding operation is fixedly set when designing the image forming apparatus. Therefore, the waiting time of the recording sheet at the pair of registration rollers is ideally constant. However, in actual image forming apparatuses, a delay in sheet feeding occurs due to a protrusion of a leading end of a recording sheet from a sheet cassette, a physical slippage occurred between a recording sheet and rollers for conveying the recording sheet, or a defect in a pickup operation performed by a pickup roller.
As a technology of suppressing such delay in sheet feeding and smoothly supplying a recording sheet to an image forming portion, for example, Japanese Unexamined Patent Publication No. 2002-116590 discloses a technology to compensate for a delay in sheet feeding for each recording sheet by controlling the timing of starting rotation of a sheet feeding roller.
However, the technology disclosed in this prior art document has the problem that a control device and related parts are subject to a great operation load to compensate for delay in sheet feeding every time before the image transfer to each recording sheet is completed.